Optical data recording medium and method for reproducing recorded data

ABSTRACT

An optical data recording medium, in which irradiation of a light beam is used for recording and/or reproducing data includes (i) a substrate having an a rise and/or a recess which are a light-incident surface, (ii) a reflective layer, provided on the light-incident surface of the substrate, for reflecting the light beam, (iii) a light absorption layer for converting, to heat, a light of the light beam to heat on the surface of the reflective layer, (iv) a reproducing layer, provided on the surface of the heat-light converting layer, having a transmittance that changes in accordance with a light intensity distribution of the light beam. The optical data recording medium is excellent in super-resolution property, and enables reproduction of a shorter mark length.

This Nonprovisional application is a Divisional application of priorcopending U.S. patent application Ser. No. 10/824,926, filed on Apr. 14,2004, entitled OPTICAL DATA RECORDING MEDIUM AND METHOD FOR REPRODUCINGRECORDED DATA by Hideharu Tajima, Nobuyuki Takamori, Go Mori and MasakiYamamoto (the same inventors as the inventors of this disionalapplication).

This Nonprovisional application claims priority under 35 U.S.C. §119(a)on Patent Application No. 2003/155668 filed in Japan on May 30, 2003,the entire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to (i) an optical data recording mediumfor recording or reproduction of data, and (ii) a method for reproducingdata from the optical data recording medium.

BACKGROUND OF THE INVENTION

In order to obtain an optical data recording medium having biggerstorage capacity, there has been a demand for a technique to record andreproduce a signal in accordance with a further shorter mark length (pitlength) in the optical data recording medium. Recently, an optical datarecording medium has been developed which enables reproduction of asignal from a mark having a mark length shorter than a mark length ofresolution limit of an optical system of a reproducing apparatus.Hereinafter, the optical data recording medium is referred as a“super-resolution optical data recording medium.”

The super-resolution optical data recording medium includes at least areproducing layer and a reflective layer layered in this order on asubstrate. The super-resolution optical data recording medium employs amethod of virtually reducing a spot size of a laser beam which reachesthe reflective layer. Specifically, the spot size of the laser beamwhich reaches the reflective layer is reduced by irradiating areproducing laser beam via the substrate to the reproducing layer. Inthis way, the reproducing layer has such an optical characteristic withrespect to the reproducing laser beam that is distributed unevenly overthe reproducing layer.

In other words, light intensity in the spot of the reproducing laserbeam irradiated to the reproducing layer is unevenly distributed.Because of this, temperature is also distributed unevenly thereinspontaneously. Therefore, in such an arrangement wherein the reproducinglayer is made of a material whose optical characteristics (mainlytransmittance) are changed by temperature or light intensitydistribution, it is possible to increase only the transmittance of thatpart of the reproducing layer which is at the center of the laser beamspot because temperature is high and light intensity is high in the partat the center of the laser beam spot. When transmittance is increasedonly in that part of the reproducing layer which is at the center of thelaser beam spot, the reflection layer receives only the light of thecenter of the laser spot. That is, the laser beam spot irradiated on thesurface of the reflective layer is virtually reduced. Therefore, in thesuper-resolution optical data recording medium, it is possible toreproduce a mark having a mark length shorter than a mark of resolutionlimit of the optical system.

As an example of a layer material whose optical characteristic ischangeable by light intensity, a shutter layer (a layer in whichsemi-conductor fine particles are dispersed in a matrix made of glass orresin) is described in Japanese Publication for Unexamined PatentApplication No. 6-28713 (Tokukaihei 6-28713, published on Feb. 4, 1994).In the arrangement of the patent application, as shown in FIG. 7, theshutter layer (reproducing layer) 42 and the optical reflective layer 44are layered on that surface of the substrate 45 which is reverse to thesurface from above which the laser beam is irradiated.

Incidentally, as an example of a raw material whose transmittance isincreased by high temperature, a thermochromic pigment is described inJapanese Publication for Unexamined Patent Application No. 2001-35012(Tokukai 2001-35012, published on Feb. 9, 2001). In the optical datarecording medium described in the patent application, as shown in FIG.8, a mask layer (reproducing layer) 32, a first dielectric layer 36, aphase change recording film 37, a second dielectric 38, a reflectivelayer 34, and a protective resin layer 39 are layered in this order onthat surface of the substrate 35 which is reverse to the surface fromabove which the laser beam 30 is irradiated.

As described above, in each conventional super-resolution optical datarecording medium, the reproducing layer is provided on that surface(non-light-incident surface) of the substrates which is reverse to thesurface (incident surface) from above which the laser beam isirradiated.

However, in the conventional optical data recording media, resolutionlimit is not enough. Therefore, there is a demand for an optical datarecording medium having a greater resolution limit.

In the arrangement in which a reproducing layer is provided on thenon-light-incident surface, the reproducing layer cannot be thickerbecause a recording layer and a reflective layer are also provided inthe optical data recording medium. Accordingly, the optical datarecording medium including a further shorter mark length cannot bereproduced.

Specifically, in the conventional optical data recording medium,generally the reflective layer has a non-flat surface (a rise and/or arecess, for example formed by pits and/or groove, or the like). When thelaser beam is irradiated to the non-flat surface, a laser beam reflectedfrom the rise part of the non-flat surface is different in quantity fromone reflected from the recess part of the non-flat surface because ofinterference. By using the difference, tracking on grooves, and signalreproduction are performed. The rise and/or the recess of the reflectivelayer is formed by forming a non-flat surface on the substrate by usinga pit and a groove, or the like, which are for storing data or forlocating a reproducing point. The reflective layer is layered on thesubstrate. Therefore, in the conventional arrangement, the reproducinglayer is provided on the substrate having the rise and/or the recess,and the recording layer and the reflective layer are layered in thisorder on the reproducing layer. Accordingly, when the reproducing layeris too thick, the rise and/or the recess are leveled off, and thus therecording layer and the reflective layer cannot have a non-flat surface.

In the above-mentioned Japanese Publication for Unexamined PatentApplication No. 6-28713, there is an example that the rise and/or therecess of the substrate are leveled off because the reproducing layerlayered on the substrate is too thick. In the example, a resin layer isused as the reproducing layer, and the resin is so adhesive that it isvery difficult to attain a thin thickness of the resin layer. Thus,there is a high possibility that the formation of the resin layer willlevel off the non-flat surface of the substrate, the rise and/or therecess being a source of data. In case the reflective film is providedon the substrate having such leveled-off non-flat surface, nointerference in the reflected light beam will be caused by theleveled-off rise and/or the leveled-off recess, whereby, data cannot beread out. Moreover, as to an inorganic film, which may have a thinthickness, the same is true that there is a possibility that a thickthickness of the inorganic film will level off the rise and/or therecess. Therefore, there is a limit in how thick the reproducing layercan be.

The arrangement in which the thickness of the reproducing layer is thinin view of the above limitation faces the following problem: forexample, in the case that the reproducing layer has a greatertransmittance with a thicker thickness, the thin thickness limits howmuch the laser spot can be reduced, thereby prohibiting the optical datarecording medium from having a better resolution limit.

Also, it is considered that the resolution limit of the optical datarecording medium is limited by various factors apart from the limitationof the thickness of the reproducing layer.

SUMMARY OF THE INVENTION

The present invention is made in light of the foregoing problems. Anobject of the present invention is to provide an optical data recordingmedium in which a signal can be reproduced from a mark having a shortermark length (that is, enables reproduction of a shorter mark length),and in which data can be recorded in high-density.

To achieve the object, an optical data recording medium, in whichirradiation of a light beam is used for recording or reproducing data,includes a reproducing layer, provided to face a light-incident surfaceof the substrate, the reproducing layer for reproduction of a signalfrom a mark having a mark having a mark length shorter than a marklength of a resolution limit of an optical system of a reproducingapparatus for reproducing the optical data recording medium.

The “reproducing layer for reproduction of a signal from a mark having amark length shorter than a mark length of a resolution limit of anoptical system of a reproducing apparatus” is a layer for reproductionof a signal from a mark having a shorter mark length smaller than alaser beam spot narrowed by the optical system of the reproducingapparatus. For example, with an arrangement in which the reproducinglayer is made of a material whose transmittance increases upon receptionof intensive light or high temperature, only a highly intensive part ofthe light beam irradiated on the reproducing layer passes through thereproducing layer, thereby giving a smaller beam spot size to the lightbeam emitted from the reproducing layer. This makes it possible toreproduce a signal from a mark having a shorter mark length than thebeam spot narrowed by the optical system of the reproducing apparatus.

In light of the characteristics of the material, the reproducing layerneeds to be provided so that the light beam is radiated via thereproducing layer to the layers such as a layer for reflecting the laserbeam. Therefore, in case where the reproducing layer is provided on anon-light-incident surface of the substrate, it is necessary that thereproducing layer be provided on the non-light-incident surface of thesubstrate, and the other layers such as the reproducing layer areprovided on a top of the reproducing layer. On the other hand, accordingto this arrangement, the reproducing layer is so provided that the lightis radiated from above the reproducing layer and the reproducing layeris the furthest from the substrate (except the cover layer), no matterhow the other layers are provided.

In other words, the reproducing layer can be formed so as to face thelight-incident surface of the substrate, after the other layers areprovided. Therefore, the reproducing layer can have an arbitrarythickness without limitation from the shape of the other layers. Forexample, in the case wherein a reproducing layer is used whose greaterthickness gives more greatly changeable transmittance thereof, thisarrangement attains a better resolution and a smaller spot size of thelight beam, thereby enabling the reproduction of the signal from a markhaving a further shorter mark length. This makes it possible to providethe optical data recording medium that is more excellent insuper-resolution property and enables storage/reproduction of data inhigher density.

Note that this arrangement attains not only freedom in designing thethickness of the reproducing layer. As described in Examples,improvement in resolution limit was observed in the optical datarecording medium having this arrangement and being identical with aconventional optical data recording medium in terms of the thickness ofthe reproducing layer, and the like condition.

To achieve the object of the present invention, the optical datarecording medium, in which irradiation of a light beam is used forreproducing data, includes steps of (i) irradiating the light beam fromabove the reproducing layer, and (ii) reproducing the mark having a marklength shorter than resolution limit of the optical system of thereproducing apparatus. On account of this, it becomes possible toreproduce data from the optical data recording medium in which data isrecorded in high-density.

For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section view of an optical data recording medium of anembodiment of the present invention.

FIG. 2( a) and FIG. 2( b) are diagrams illustrating irradiation of areproducing laser beam to the optical data recording medium of anembodiment of the present invention. FIG. 2( a) is a plain viewillustrating a temperature distribution of the reproducing laser beam inthe irradiated spot. FIG. 2( b) is a diagram illustrating (i) crosssection of the optical data recording medium and (ii) a temperaturedistribution.

FIG. 3 is a diagram illustrating how the reproducing laser beam isirradiated to the optical data recording medium of an embodiment of thepresent invention.

FIG. 4 is a cross section view of an optical data recording medium of acomparative example of the present invention.

FIG. 5 is a graph comparing (i) an optical data recording mediumrelating to an example of the present invention to (ii) the comparativeexample, in terms of dependency of C/N on mark length.

FIG. 6 is a graph illustrating the dependency of C/N on mark length ofthe optical data recording medium of the example of the presentinvention.

FIG. 7 is a cross section view of a conventional optical data recordingmedium.

FIG. 8 is a cross section view of another conventional optical datarecording medium.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, an embodiment of the present invention is explained withreference to figures.

An optical data recording medium 31 of one embodiment of the presentinvention is a reproducing only optical data recording medium. As shownin FIG. 1, the optical data recording medium 31 is provided with areflective layer 4, a light absorption layer 3, a reproducing layer 2, asubstrate 5, and a cover layer 1. The reflective layer 4, lightabsorption layer 3, reproducing layer 2 are layered on the substrate 5in this order. On the reproducing layer 2, the cover layer 1 isprovided. In the optical data recording media 31, the cover layer 1 doesnot completely adhere to the reproducing layer 2, and a layer of air isformed between the cover layer 1 and the reproducing layer 2.

A laser beam 30 is irradiated, from above the cover layer 1, to theoptical data recording medium 31. The laser beam passes through thecover layer 1 and the layer of air, and reaches the reproducing layer 2.Then the laser beam reaches the reflective layer 4 via the reproducinglayer 2 and light absorption layer 3. The laser beam 30 reflected fromthe reflective layer 4 is converted to a signal corresponding to therise and/or the recess of the reflective layer 4.

The substrate 5 gives appropriate strength to the optical data recordingmedium 1. On an light-incident surface of the substrate 5 (that surfaceof the substrate 5 from above which the laser light beam is irradiated;that is, that surface of the substrate 5 above which the reproducinglayer 2 is provided), pits and grooves are provided. The pits, whichform the rise and/or the recess, correspond to recorded data, and thegrooves are used for guiding, that is, for recording a start address andend address of recording. The optical data recording medium 31 may beprovided with both the pits and the grooves, or with either the pits orthe grooves. However, in the arrangement in which the optical datarecording medium 1 includes the guiding grooves, data can be recorded orreproduced without imposing a burden on a reproducing/recording device.

Optical characteristics of the material of which the substrate 5 is madeare not particularly limited. Thus, the material of which the substrate5 is made can be transparent or opaque. The material may be, forexample, (i) glass, (ii) a thermo-flexible transparent resin such as apolycarbonate, an amorphous polyolefin, a thermoplastic polyimide, PET(polyethylene terephtalate), PEN (polyethylene naphthalate), and PES(polyether sulfone), (iii) a thermo-cured transparent resin such as athermo-cured polyimide, and an ultraviolet radiation cured acrylicresin, (iv) a metal, or (v) the like. The substrate 5 may be made of oneof these materials solely or two or more of these materials used incombination. Also, thickness of the substrate 5 is not particularlylimited. However around 0.5 mm through 1.2 mm, for example, isappropriate. A pitch of the pit may be, for example, around 0.3 μmthrough 1.6 μm, and a depth of the pit may be, for example, around 30 nmthrough 200 nm. It is appropriate for a pitch of the guiding groove tobe around 0.3 μm through 1.6 μm, and it is appropriate for depth of theguiding groove to be around 30 through 200 nm.

The reflective layer 4 reflects the laser beam which has passed throughthe cover layer 1, the reproducing layer 2, and the light absorptionlayer 3. Here, the reflective layer 4 has a non-flat surface (a riseand/or a recess) because a reflective film 4 is provided on the non-flatsurface of the substrate 5. A reproducing signal is produced inaccordance with quantity of the light beam reflected from the reflectivelayer 4 (the quantity of the laser beam is variable depending on therise and/or the recess in a laser-beam-irradiated area of the reflectivelayer 4).

It is preferable that the reflective layer 4 is a metal film having highreflectance such as an Al film, an Au film, an Ag film, or a film of analloy of those compounds. Because the reflective layer 4 is notparticularly limited in thickness, it may have any thickness to realizea desired reflectance. For example, the thickness of the reflectivelayer 4 may be around 20 nm through 100 nm.

The light absorption layer 3, by receiving the reproducing laser beam30, assists temperature change in the reproducing layer 2. The lightabsorption layer 3 is made of a material which absorbs the reproducinglaser beam 30 and converts the laser beam to heat. The light absorptionlayer 3 changes its temperature in accordance with the light intensitydistribution, and conducts to the reproducing layer 2, heat thusgenerated.

The light absorption layer 3 may be a Si (silicon) film; a Ge(germanium) film; a phase change film such as a AgInSbTe film and aGeSbTe film, and the like; a magnet-optical film such as TbFeCo film,DyFeCo film, GdFeCo film, and the like; and a metal film of an alloy ofthese compounds. Especially, it is the most preferable that the lightabsorption layer 3 is the Si film, the Ge film, or a metal film of alloyof Si and Ge, because of their low cost. Film thickness of the lightabsorption layer 3 may be set to be appropriate depending on whichmaterial the light absorption layer 3 is made of. For example, it isappropriate that the light absorption layer 3 has a thickness in a rangeof 5 nm through 300 nm. However, it is preferable that the filmthickness of the light absorption layer 3 is no less than 10 nm.Therefore, it is the most preferable that the light absorption layer 3is the Si film having a thickness of no less than 10 nm.

It is preferable that the reproducing layer 2 is contiguous to the lightabsorption layer 3, as shown in FIG. 1. With this arrangement, the lightabsorption layer 3 can effectively raise the temperature of thereproducing layer 2 by absorbing, for example, the reproducing laserbeam 30 and converting the beam to heat. However, the reproducing layer2 may be so arranged as not to be contiguous to the light absorptionlayer 3, as long as the light absorption layer 3 and the reproducinglayer 2 are close enough to allow the light absorption layer 3 toincrease the temperature of the reproducing layer 2.

In addition, the optical data recording medium 31 may be so arranged asnot to include the light absorption layer 3. In this case, however, thereproducing layer 2 must be made of a material whose opticalcharacteristic is changed only by light intensity, or the reproducinglayer 2 must have light-heat converting function by containing asubstance which absorbs the reproduction light and generates heat.

The reproducing layer 2, which is a translucent material whosetransmittance is changed reversibly as a temperature changes, contains amaterial whose transmittance with respect to a wavelength of thereproduction laser beam 30 increases as a temperature rises. With thisarrangement, transmittance is increased only in a temperature-risingpart of the laser beam spot of the reproduction laser beam 30 (a smallerspot near at a center of the reproduction beam 30). Accordingly, thediameter of the laser beam spot of the laser beam having passed throughthe reproducing layer 2 becomes smaller than the diameter of the spot ofthe reproducing laser beam 30. On account of this, it is possible toperform reproduction of a shorter mark length.

It is appropriate that the reproducing layer 2 includes a material whosetransmitting efficiency in specified wavelength area greatly changeswhen temperature rises. Specifically, the reproducing layer 2 preferablyincludes a material whose transmitting efficiency of the reproducinglayer 2 changes in the range of ±80% when temperature rises from 20° C.to a temperature in a range of 60° C. through 180° C. The material maybe a thermochromism substance. The thermochromism substance is asubstance whose transmittance changes due to a chemical structuralchange caused by heat absorption.

It is possible to raise specific examples of the thermochromismsubstance as follows: (i) an inorganic thermochromism substance such asmetal oxides and the like; and (ii) an organic thermochromism substancesuch as (a) a mixture of (a-1) lactone or fluorane and (a-2) an alkalis,(b) a mixture of leuco pigment and organic acid, and (c) the like. It isparticularly preferable that, from among those substances, thethermochromism substance is a metal oxide whose transmittance of theabsorption edge changes in accordance with a change in its width offorbidden band. The change in the width of forbidden band is caused bytemperature change. This is because composition and shape of thereproducing layer 2 made of a metal oxide are hardly changed even afterchemical structural changes due to temperature change are repeated. Inother words, this metal oxide gives excellent durability to thereproducing layer 2.

It is possible to raise specific examples of the metal oxide as follows:ZnO, SnO₂, CeO₂, CeO₂, NiO₂, In₂O_(3, TiO) ₂, Ta₂O₅, VO₂, SrTiO₃, andthe like. Of these metal oxides, it is the most preferable that thereproducing layer 2 is made of ZnO (zinc oxide) as described in Example2 below. This is because reproduction from a mark having a furthershorter mark length is possible in case that the reproducing layer 2 ismade of ZnO. The reproducing layer 2 may be made of a conventionalmaterial for reproducing layers. Examples of the conventional materialfor reproducing layers are: a glass having semi-conductor fine grain; aresin; a thermochromic pigment layer; a phase change film; and the like.

Film thickness of the reproducing layer 2 may be set depending on whichmaterial the reproducing layer 2 is made of. The film thickness of thereproducing layer 2 may be in a range of from 5 nm through 800 nmappropriately, and it is more appropriate that film thickness of thereproducing layer 2 is no less than 100 nm. Therefore, it is the mostappropriate that the reproducing layer 2 is the ZnO film whose filmthickness is no less than 100 nm.

The cover layer 1 is provided so that the optical system of the presentembodiment of the present invention is the same as the optical system ofthe arrangement shown in FIG. 7. Generally, the cover layer 1 isprovided to protect the optical data recording medium 31. It ispreferable that film thickness of the cover layer 1 is in a range offrom 1 μm through 100 μm. Also, the cover layer 1 needs to betransparent so that the reproducing laser beam 30 can pass through thecover layer 1.

In this arrangement, the reflective layer 4, and the light absorptionlayer 3 are layered in this order on the surface of the substrate. Onthe top of the light absorption layer 3, the reproducing layer 2 islayered. In this way, the reproducing layer 2 is the top layer of thelaminated layers (the layer which is the furthest from the surface ofthe substrate except the cover layer).

Therefore, the reproducing layer 2 can have an arbitrary thicknesswithout limitation from the shape of the reflective layer 4 because thereproducing layer 2 is formed after the formation of the reflectivelayer 4 having a rise and/or a recess faithful with the rise and/or therecess of the substrate 5. This arrangement attains a good transmittancedistribution along a thickness direction, whereby it becomes possible toperform the reduction of signals from the shorter mark length.Therefore, with this arrangement, it is possible to attain a highersuper resolution property and to realize an optical data recordingmedium in which a signal can be recorded in high density, and the signalrecording in high density can be reproduced.

Incidentally, Blu-ray disc (BD) may be so arranged that a recordingsurface is provided on a light-incident surface of a substrate. In thistype of recording medium, a laser beam can be irradiated to therecording surface without passing through the substrate. Therefore thelaser beam can be irradiated from closer range than when in thearrangement in which the laser beam has to pass through the substrate.On the account of this, the laser beam having smaller spot size can beirradiated by using a lens having a high NA. With this arrangement, itis possible to attain reproduction of a signal from a mark having ashorter mark length. However, even though the recording layer is thusprovided on the light-incident surface of the substrate, there is alimit in how much the distance between the recording surface and thelaser irradiation point can be short. In this case, the arrangement inwhich the reproducing layer is provided on the light-incident surface ofthe substrate as in the present embodiment makes it possible toreproduce an optical data medium having a further shorter mark length.

Hereinafter, a method of reproducing the optical data recording medium31 is explained with reference to FIGS. 2( a) and FIG. 2( b)

The reproduction in the optical data recording medium 31 can be carriedout by detecting the light beam reflected from the light-incidentsurface of the substrate 5 by using an optical head (not shown) as aresult of the irradiation of the reproducing laser beam 30 onto thelight-incident surface from above the cover layer 1 by using a laserlight source (not shown) and an optical system (such as a condenserlens). On the incident surface at least either pit or groove areprovided.

Here, the irradiation of the reproduction beam 30 onto the optical datarecording medium 31 is carried out in such a manner that an area havinghigher temperature and an area having lower temperature are produced inthe laser beam spot of the reproducing layer 2. For example, when thereproducing laser beam 30 is irradiated from above the cover layer 1 tothe reproducing-only optical data recording medium 31, the reproducinglaser beam spot 11 is produced on the surface of the reproduction layer2. The reproducing laser beam spot 11 has a temperature gradient fromthe center of the spot to the other area of the spot as shown in theFIG. 2( a). Therefore, a higher temperature area 13 and a lowertemperature area 12 appear in the reproducing laser beam spot 30 on thesurface of the reproducing layer 2. For example, the higher temperaturearea 13 has a temperature not less than 60° C. but less than 180° C.,and the lower temperature area 12 has a temperature not less than —20°C. but less than 60° C. i.e., when the reproducing laser beam 30 isirradiated to the optical data recording medium 31, temperature is thehighest in the center of the laser beam spot, and a part further fromthe center an area has a lower temperature.

Transmittance of the reproducing layer 2 changes in accordance withtemperature changes. Therefore, the transmittance of the reproducinglayer 2 for wavelength of the reproducing laser beam 30 decreases (lowtransmittance state) in the higher temperature area 13 where temperaturerises due to irradiation of the reproducing laser beam 30. On the otherhand, the transmittance of the reproducing layer 2 for wavelength of thereproducing laser beam 30 does not decrease in the lower temperaturearea 12 where temperature does not rise very much even though thereproducing laser beam 30 is irradiated thereto.

Accordingly, most of the laser beam irradiated to the optical datarecording medium 31 (the laser beam irradiated to the lower temperaturearea 12) are shielded off by the reproducing layer 2, and only the laserbeam irradiated to the higher temperature area 13 passes through thereproducing layer 2 as shown in FIG. 3. On account of this, only thelaser beam having passed through the reproducing layer 2 reaches thelight absorption layer 3 and the reflective layer 4. Therefore, the spotsize of the laser beam produced on a surface of the reflective layer 4is virtually reduced. Consequently, it is possible to performreproduction of a mark having a mark length shorter than a mark lengthof the resolution limit of the optical system.

Note that when the reproducing laser beam 30 having the highertemperature area and the lower temperature area is irradiated, the lightabsorption layer 3 absorbs the reproducing laser beam 30 and convertsthe beam into heat. Therefore, the light absorption layer 3 produces alarge amount of heat after absorbing the reproducing laser beam 30having passed through the higher temperature area 13. Because the heatgenerated in the light absorption layer 3 travels to the reproducinglayer 2 located nearby (preferably contiguous to) the light absorptionlayer 3, temperature in the higher temperature area 13 of thereproducing layer 2 rises more. Accordingly, the transmittance of thelaser beam irradiated to the higher temperature area 13 in a reproducinglayer 2 increases more. This makes it easier to attain a further smallerspot size of the laser beam on the reflective layer 4, thereby attainingreproduction of higher quality.

The optical data recording medium having the reproducing layer of theembodiment of the present invention may be, but not limited to adisc-shaped optical data recording medium such as CDs (Compact Discs),CD-ROMs (Compact Disc-Read Only Memorys), CD-Rs (CompactDisc-Recordables), CD-RWs (Compact Disc-ReWritables), DVDs, DVD-ROMs,DVD-Rs, DVD-RWs, DVRs (Blu-ray Discs), DVR (Blu-ray Disc)-ROMs, and thelike.

Also, the arrangement of the present invention is not limited to theoptical data recording medium 31. For example, instead of providing thecover layer 1 made of glass on a surface of the reproducing layer 2, theoptical data recording medium may be so arranged that a resin layerhaving refraction index and heat conductivity similar to those of theair is provided contiguously on the reproducing layer 2. (The resinlayer must be absolutely adhered to the reproducing layer 2.) Because aresin having smaller refraction index and heat conductivity than the airhas does not exist at the present moment, it is believed that a maximumperformance can be obtained by the arrangement in which the reproducinglayer is provided contiguously to a layer of air.

Moreover, the optical data recording medium may be so arranged as not toinclude the reflective layer, if that surface of (a reflective surface)the substrate 5 which faces to the reproducing layer 2 has enoughreflectivity.

The optical data recording medium may be so arranged that the riseand/or the recess, those of which indicates a start address of data andan end address of data are not provided. However, in this case, theoptical data recording medium does not indicate where recording pointand/or reproducing point are.

Note that the embodiment of the present invention includes a recordableoptical recording medium.

In order to obtain the recordable optical data recording medium, thelight absorption layer 3 of the embodiment is so arranged as to be madeof a material with which the light absorption layer 3 becomesrecordable, so as to have functions of converting light into heat and ofrecording data, thereby causing the optical data recording medium to berecordable. It is possible to raise examples of the materials with whichthe light absorption layer 3 becomes recordable as follows: a phasechange recording material (such as GeSbTe, or the like), a pigment, anda magnet-optical recording material (such as TbFeCo, or the like).

As described above, that optical data recording medium of the presentinvention, in which irradiation of a light beam is used for recording orreproducing data, includes a reproducing layer, provided to face alight-incident surface of the substrate. The reproducing layer is forreproduction of a signal from a mark having a mark having a mark lengthshorter than a resolution limit of an optical system of a reproducingapparatus for reproducing the optical data recording medium.

The “reproducing layer for reproduction of a signal from a mark having amark having a mark length shorter than a resolution limit of an opticalsystem of a reproducing apparatus for reproducing the optical datarecording medium” is a layer which is provided to reproduce a marklength shorter than the laser beam spot which is narrowed by the opticalsystem of the reproducing apparatus. For example, only highertemperature area of the laser beam, which is irradiated to thereproducing layer, passes through the reproducing layer if a materialwhich increases its transmittance when an intensive laser beam isirradiated or when temperature increases is used for the reproducinglayer. As a result, the spot size of the laser beam is reduced.Accordingly, it becomes possible to reproduce a mark length shorter thanthe laser beam spot which is narrowed by the optical system of thereproducing apparatus.

In light of the characteristics of the material, the reproducing layerneeds to be provided above the layers—for example, such as a layer whichreflects the laser beam—on the light-incident surface of the substrate.Therefore, when the reproducing layer is provided on a reverse surfaceto that surface of the substrate from above which the laser beam isirradiated, the reproducing layer needs to be provided firstly on thesurface of the substrate. Then, above the reproducing layer, the otherlayers are provided. On the other hand, according to the foregoingarrangement, it is possible to provide the reproducing layer on the topof layers (except the cover layer) on that surface of the substrate fromabove which the laser beam is irradiated, after the other layers, forexample a layer for reflecting the laser beam, are provided.

Because the reproducing layer can be provided on that surface of thesubstrate from above which the laser beam is irradiated after the otherlayers are provided, it is possible to give the reproducing layer anarbitrary thickness, while letting the reflecting layer have a good riseand/or a good recess. On account of this, resolution is improved in casewhere the reproducing layer is made of a material whose transmittance isgreatly changed by the thicker film thickness. Also, because the size ofthe laser beam becomes smaller, it becomes possible to reproduce ashorter mark length. Accordingly, because super-resolution quality isimproved, an optical data recording medium in which data is recorded inhigh-density is obtained.

Note that in the arrangement, not only the film thickness of thereproducing layer is not limited but also resolution limit is improvedwhen a conventional optical data recording medium and film thickness ofthe reproducing layer are arranged as above. This is described later inExamples.

To achieve the object of the present invention, the optical datarecording medium includes (i) the substrate having the rise and/or therecess that contributes recording and/or reproduction on the lightincident surface, (ii) functional layers, provided on the light incidentsurface of the substrate, assisting recording and reproducing data, and(iii) the reproducing layer, provided on the surface of the functionallayers, having transmittance that changes in accordance with a lightintensity distribution of the laser beam.

“The functional layers assisting recording and reproducing data” are oneor more layers, which have functions of reflecting the laser beam,converting light to heat, recording data, or the like function. Eachlayer may have a single function or multiple functions. This is, thefunctional layer may have the function of converting light to heat andthe function of recording data.

The reproducing layer needs to be provided so that the light is radiatedvia the reproducing layer to the functional layer. Therefore, in casewhere the reproducing layer is provided on the reverse surface to thenon-light-incident surface of the substrate, it is necessary that thereproducing layer be provided on the non-light-incident surface of thesubstrate, and the functional layer is provided on the reproducinglayer. On the other hand, according to the foregoing arrangement, thereproducing layer is provided on the top of layers layered on thesubstrate (the furthest layer from the substrate except the coverlayer).

Therefore, with this arrangement, it is possible to give the reproducinglayer an arbitrary thickness, while letting the reflecting layer have agood rise and/or a good recess. On account of this, resolution isimproved in those cases wherein the reproducing layer is made of amaterial whose transmittance is greatly changed by the thicker filmthickness. Also, it becomes possible to reproduce a shorter mark length.Accordingly, this gives the optical data recording medium a highersuper-resolution property, and a high-density optical data recordingmedium is obtained.

Note that it is desirable that “the rise and/or the recess indicative ofdata or reproduction position” formed on the substrate includes a groovefor recording a start address and an end address of the data, inaddition to the pit or groove for recording the data. (The groove forrecording the start address and end address of data indicates, afterrecording the data, where the reproduction point of the data is.) Withthis arrangement, the recording and reproducing of the data can becarried out without imposing a burden on the reproducing/recordingapparatus. Thus, it is possible to reproduce the recorded data in higherdensity.

To achieve the object of the present invention, the optical datarecording medium is so arranged that the reproducing layer includes amaterial whose transmittance changes in accordance with temperature.Because the transmittance of the reproducing layer changes in accordancewith temperature, it becomes possible to attain a smaller size of thelight beam spot desirably.

To achieve the object of the present invention, the optical datarecording medium, at least a part of that surface of the reproducinglayer to which the light beam is irradiated is exposed to air.

According to the arrangement, because at least a part of the surface ofthe reproducing layer to which the laser beam is irradiated is exposedto air, it is possible to attain a desirable difference betweenrefractive index of the air and refractive index of the reproducinglayer in irradiating the light beam onto the reproducing layer. On theaccount of this, it becomes easier to irradiate the laser beam to thereproducing layer.

Furthermore, in the arrangement in which the recording layer has atransmittance that is changeable by heat distribution caused by thelight beam, conduction of the heat from the reproducing layer to theother layers is minimized. Thus, it is possible to heat the reproducinglayer by the light beam efficiently.

Therefore, according to the foregoing arrangement, not only is itpossible to prevent heat from transmitting from the reproducing layer,but also it is possible to increase a quantity of the reflected lightbeam. Accordingly, it is possible to obtain the optical data recordingmedium in which data is recorded in high density can be reproduced withbetter quality.

To achieve the object of the present invention, the optical datarecording medium includes that light absorption layer for converting thelight beam to heat, which is contiguous to the reproducing layer.

In those cases wherein an optical data recording medium of the presentinvention does not include the light absorption layer, the reproducinglayer must have an light-heat converting function. In order to providean light-heat converting function with the reproducing layer, thereproducing layer should be made of (i) a material whose opticalcharacteristics are changeable only in accordance with light intensitydistribution, or (ii) a material which converts light to heat.

On the other hand, according to the foregoing arrangement, the lightbeam having passed through the reproducing layer can be converted intoheat thereby changing the temperature of the reproducing layer by thelight beam efficiently with such a simple arrangement. Therefore, it ispossible to change the temperature of the reproducing layer withoutproviding a variety of functions with the reproducing layer. On theaccount of this, a super-resolution optical data recording medium, whichcosts less and is easier to be fabricated, can be obtained.

To achieve an object of the present invention, the optical datarecording medium includes a reflective layer for reflecting the lightbeam as one of the functional layers, which is provided between thesubstrate and the reproducing layer.

In the arrangement in which the reflective layer is provided between thesubstrate and the reproducing layer, the reproduction layer is formedafter the reflecting layer is formed so as to have the rise and/or therecess well corresponding to the rise and/or the recess of thereflecting layer.

Therefore, with this arrangement, it is possible to give the reproducinglayer an arbitrary thickness, while letting the reflecting layer have agood rise and/or a good recess. As a result, the transmittance of thereproducing layer is distributed desirably in a thickness direction ofthe reproducing layer, thereby making it possible to attain thereproduction of the signal from the shorter mark length. This gives theoptical data recording medium a higher super-resolution property.

Because the reflective layer is provided in the optical data recordingmedium, the optical data recording medium can be effectively reproducedeven when the reproducing layer does not have enough reflectance. Thus,the super-resolution optical data recording medium which costs less andpossesses high reliability is obtained.

To achieve the object of the present invention, the optical datarecording medium includes the reproduction layer that is made of a metaloxide. Because the reproducing layer is made of a metal oxide, thesuper-resolution optical data recording medium of the present inventioncosts less and possesses higher reliability than heretofore was thecase.

To achieve the object of the present invention, the optical datarecording medium includes the reproducing layer that is made of a zincoxide. Because the reproducing layer is made of zinc oxide, it ispossible to read the non-flat surface having shorter mark length and towrite data in high density in the optical data recording medium.

To achieve an object of the present invention, the optical datarecording medium includes a light absorption layer made of one ofsilicon, germanium or an alloy of silicon and germanium. Because thelight absorption layer is made of one of silicon, germanium or an alloyof silicon and germanium, it is possible to attain an optical datarecoding medium having a reproducing layer whose temperature can bechanged desirably by using the light beam, while keeping the low cost ofthe optical data recording medium.

To achieve the object of the present invention, the reproducing methodof an optical recording medium includes the steps of (i) irradiating thelaser beam from above the reproducing layer, and (ii) reproducing themark having a mark length shorter than resolution limit of the opticalsystem of the reproducing apparatus. On the account of this, it becomespossible to reproduce data recorded in the high-density optical datarecording medium.

The invention being thus described, it will be obvious that the same maybe varied in many ways. All such modifications as would be obvious toone skilled in the art are intended to be included within the scope ofthe following claims.

Furthermore, the present invention can be structured as follows.

A first optical data recording medium, in which irradiation of a lightbeam is used for recording or reproducing data, at least includes (i) asubstrate and (ii) a reproducing layer, provided to face alight-incident surface of the substrate, for reproduction of a signalfrom a mark having a mark having a mark length shorter than a resolutionlimit of an optical system of a reproducing apparatus for reproducingthe optical data recording medium, wherein the reproducing layer isprovided on a surface of the substrate to which a laser beam isirradiated.

A second optical data recording medium is, in addition to thearrangement of the first optical data recording medium, arranged suchthat the substrate has a rise and/or a recess on its surface that is toface the reproducing layer, the rise and/or the recess contributing torecording and/or reproduction of data.

A third optical data recording medium is, in addition to the arrangementof the first or second optical data recording medium, wherein at least apart of the surface of the reproducing layer, to which the laser beam isirradiated, of the first or the second optical data recording medium isexposed to the air.

A fourth optical data recording medium, in addition to the arrangementof any one of the first through the third optical data recording media,includes an light absorption layer between the reproducing layer and thesubstrate.

A fifth optical data recording medium, wherein the light absorptionlayer of the forth optical data recording medium is made of a silicon ora germanium or an alloy of a silicon and a germanium.

A sixth optical data recording medium, wherein the reproducing layer ofany one of the first through the fifth optical data recording media ismade of a metal oxide.

A seventh optical data recording medium, wherein the reproducing layerof any one of the first through the fifth optical data recording mediais made of a zinc oxide.

An eighth optical data recording medium, wherein any one of the firstthrough the seventh optical data recording media includes a reflectivelayer between the light absorption layer and the substrate.

A reproducing method of any one of the first through the eighth opticaldata recording media includes a step of reproducing a shorter marklength signal than resolution limit of optical system of the reproducingapparatus.

Examples Example 1

As an Example 1, an optical data recording medium having the followingarrangement was produced (hereinafter, referred as “Example 1 disc”). Asshown in the FIG. 1, pits creating a non-flat surface are provided onpolyolefin-based resin substrate 5 having a 0.5 mm thickness. The pitscorresponded to recorded data. On that surface of the polyolefin-basedresin substrate 5 on which the pits are formed, an Al layer 4 (30 nm inthickness) used as a reflective layer, as Si layer 4 (50 nm inthickness) used as an light absorption layer, and a ZnO film 2 (225 nmin thickness) used as a reproducing layer were formed in this order. Ona top surface of the reproducing layer 2, glass 1 (0.5 mm in thickness)as a cover layer was placed.

Also, as a Comparative Example, an optical data recording medium withfollowing arrangement was produced (hereinafter referred as“conventional disc”). As shown in FIG. 4, pits creating a non-flatsurface were provided on a polyolefin-based resin substrate 25 having0.5 mm thickness. On the surface of the polyolefin-based resin substratehaving the pits, a ZnO film 22 (225 nm in thickness) used as areproducing layer, a Si layer 23 used as an light absorption layer 23(50 nm in thickness), and an Al layer 24 (30 nm in thickness) werelayered in this order.

By using the Example 1 disc and the conventional disc, the correlationbetween a mark length and signal quality was measured. In themeasurement, a wavelength of a reproducing laser beam 30 was set at 408nm, an aperture NA of a lens was 0.65, and a linear velocity of scanningthe reproducing laser beam was set at 3.0 (m/s).

Note that in both the discs, the layers having the same functions wereidentical in material and in thickness, in order to perform thecomparison between the discs more accurately. Moreover, because the samemeasuring apparatus was used to compare the Example 1 disc and theconventional disc (so that optical systems until the light reached thereproducing layers were identical in both the discs), the conventionaldisc included a glass 1 which was as thick as the glass 1 which theExample 1 disc included.

As to the Example 1 disc, measurement of a C/N (appraisal standard ofsignal quality) of pits having 0.1 μm through 0.5 μm mark length (pitlength), and C/N obtained by irradiation of the reproducing laser beam30 onto the Example 1 disc from above the glass. The result is graphedin a solid line in FIG. 5. In FIG. 5, the horizontal axis shows the pitlength, and the vertical axis is OTF (optical transfer function) showingC/N (appraisal standard of signal quality) and dependency of C/N onrecording mark length. In other words, the vertical axis showssuper-resolution quality.

As to the conventional disc, a measurement of a C/N (appraisal standardof signal quality) of a pit having 0.1 through 0.5 μm mark length (pitlength) was carried out. The result is graphed in a broken line in FIG.5.

According to FIG. 5, the Example 1 disc had a very high C/N values of 40to 45 dB for the mark lengths (pit length) down to about 0.14 μm. (Inaddition, even for the pit length shorter than 0.14 μm, the C/N value ofthe Example 1 disc were still high, for example, the C/N value was 35 dBwhen the pit length was around 0.12 μm). In general, a C/N value needsto be no less than 40 dB in order to reproduce data finely. Therefore,in Example 1 disc used, it was possible to reproduce data finely for thepit lengths down to 0.14 μm. On the other hand, in the conventionaldisc, the C/N value decreased dramatically for the mark length shorterthan 0.2 μm.

For the pit length 0.14 μm, the C/N of the Example 1 disc was around 40dB and the C/N of the conventional disc was 17 dB. Also, for the pitlength 0.2 μm or less, the C/N of the conventional disc decreaseddramatically to a value lower than 40 dB. Therefore, for theconventional disc, the limit of the pit length was 0.20 μm for finereproduction. As described above, the comparison showed that thearrangement of the present invention attained dramatically highsuper-resolution quality and the reproduction of the signal from theshorter mark length with high signal quality.

Example 2

In Example 2, a material of the reproducing layer was examined.

An optical data recording medium (hereinafter Example 2 disc) used inExample 2 was identical with the Example 1 disc in the Example 1, exceptthat the reproducing layer of the Example 2 disc was made of, instead ofZnO, SnO₂. Correlation between mark lengths for signals, and qualitiesof the signals was measured for the Example 1 disc and Example 2 disc.The measurement was carried out as in Example 1. That is, themeasurement of a C/N (appraisal standard of signal quality) of pitshaving 0.1 μm through 0.5 μm mark length (pit length) was carried out.

The results of the measurements of the Example 1 disc and the Example 2disc are graphed in FIG. 6. In FIG. 6, the solid line is the result ofthe Example 1 disc, whereas the broken line is the result of the Example2 disc. In FIG. 6, the horizontal axis shows the pit length, and thevertical axis is OTF (optical transfer function) showing C/N (appraisalstandard of signal quality) and dependency of C/N on recording marklength. In other words, the vertical axis shows super-resolutionquality.

According to FIG. 6, especially for the pit length of 0.14 μm or less, aC/N of the Example 1 disc made of ZnO was 5 to 10 dB higher than a CN ofthe Example 2 disc made of SnO₂. This explains that the arrangement inwhich the reproducing layer is made of ZnO enables reproduction of asignal from a mark having a shorter mark length.

The invention being thus described, it will be obvious that the same waymay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1-18. (canceled)
 19. A write once read many optical data recordingmedium in which irradiation of a light beam is used for recording dataand reproducing recorded data, the optical data recording mediumcomprising: a substrate; a reproducing layer, provided on a lightincident side of the substrate, for reproduction of a signal from arecording mark having a mark length shorter than a resolution limit ofan optical system of a reproducing apparatus; and grooves, provided onthe light incident side of the substrate, for recording the recordingmark having a mark length shorter than the resolution limit of theoptical system of the reproducing apparatus, and for recording anaddress that provides data of a reproducing point.
 20. A method forreproducing data from an optical data recording medium as set forth inclaim 1, comprising the step of reproducing the signal from therecording mark having a mark length shorter than the resolution limit ofthe optical system of the reproducing apparatus by irradiating a lightbeam to the optical data recording medium at a side where thereproducing layer is provided on the substrate.
 21. A method forrecording data in an optical data recording medium as set forth in claim1, comprising the step of recording an address in a groove for recordingan address that provides data of a reproducing point.